The energy required to actuate a flow control valve is directly related to the difference between the fluid pressures acting on opposite sides of the valve sealing member, and to the built-in spring forces (if any) that act to maintain the sealing member in its opened or closed position. Previously, efforts have been made to provide control valves that balance fluid pressures on opposite sides of the sealing member in order to minimize the power required for actuation. For example, U.S. Pat. No. 3,985,333 shows valves having a pair of diaphragms employed to balance the fluid forces. Such diaphragms impose a limit on the fluid pressure that can be controlled and they are costly in time and effort to replace. Some other control valves often require an unacceptably long time to open and/or close.